The present invention relates to a liquid crystal display and an improvement in the contrast of a projection-type liquid crystal display using a reflection-type liquid crystal display element.
A projection-type liquid crystal display of a conventional type produces a display by projecting light onto a screen, so that it needs to produce a brightness sufficient for the display to be recognized under the normal level of illumination light present in a house. In order to enhance the brightness of the display without increasing the power consumption, it is also necessary to increase the aperture ratio. For an increase in the aperture ratio, a reflection-type liquid crystal display element in which the upper part of a TFT (Thin Film Transistor) is covered by a reflection electrode and can be also used for display is suitable. In a conventional projection-type liquid crystal display, a twisted nematic liquid crystal is used for a liquid crystal layer, and two polarizing plates are disposed in front of and behind the liquid crystal layer to produce the display.
However, since the aperture ratio is low, a sufficient brightness to make it possible to recognize the display under the normal level of indoor illumination has not been obtained. Even if a polarization beam splitter is used in place of the polarizing plate, sufficient brightness cannot be obtained. According to Japanese Patent Application Laid-Open No. 64-7021, a reflection plate is disposed between the liquid crystal layer and the lower substrate. The liquid crystal display having such a construction will be referred to herein as a reflection plate type liquid crystal display apparatus.
In the conventional reflection plate type liquid crystal display apparatus, since the wavelength dispersion of a polarization state of light transmitted through the liquid crystal layer is large, the polarization state of the light passing through the liquid crystal layer has to be controlled for the whole visible wavelength in order to obtain a high-contrast display.
However, a method of controlling the wavelength dispersion is not described in Japanese Patent Application Laid-Open No. 64-7021. Since the reflection plate type liquid crystal display has a high aperture ratio for the reason mentioned above, it has the potential ability to obtain a brightness which is sufficient to allow one to easily recognize the display under the normal level of indoor illumination. Consequently, there has been a desire to obtain sufficient brightness and contrast ratio at the same time.
It is, therefore, an object of the invention to provide a liquid crystal display having excellent visibility in which a sufficient brightness and contrast ratio are provided at the same time.
A liquid crystal display achieving the above object and having a projection optical system in which a polarizing plate, a phase plate, a liquid crystal layer, and a reflection plate are arranged in parallel and a transmitted light is reflected by the reflection plate is characterized in that the phase plate comprises first and second phase plates in which a retardation of each of the phase plates and a gradient of wavelength dispersion of the retardation can be freely set, and the gradient of the first phase plate is set so as to be larger than the gradient of the liquid crystal layer and the gradient of the second phase plate is set so as to be smaller than the gradient of the liquid crystal layer, and wavelength dispersion of a retardation composite value in the projection optical system, which is a combination of the liquid crystal layer and the first and second phase plates, is brought asymptotically closer to a retardation curve expressed by the relational expression (0.25+0.5n)xcex.
According to another aspect of the invention, there is provided a liquid crystal display in which a polarizing plate, first and second phase plates, an upper substrate, a liquid crystal layer, a drive device, and a lower substrate also serving as a reflection plate are sequentially laminated in a travelling direction of transmitted light, and an absorption axis direction of the polarizing plate and an orientation direction of the liquid crystal layer, which is on the side adjacent to the polarizing plate have a non-parallel relation, wherein each of the first and second phase plates, respectively, has a wavelength dispersion of a retardation such that a wavelength dispersion of a retardation composite value of the phase plates and the liquid crystal layer in a range from 400 (nm) to 700 (nm) of a wavelength of the transmitted light is substantially approximated to a retardation curve expressed by the relational expression (0.25+0.5n)xcex.
Further, according to still another aspect of the invention, there is provided a liquid crystal display in which a polarizing means, first and second phase plates, an upper substrate, a liquid crystal layer, a drive device, and a lower substrate also serving as a reflection plate are sequentially laminated in a travelling direction of transmitted light, and an absorption axis direction of the polarizing means and an orientation direction on the side adjacent to the polarizing means of the liquid crystal layer have a non-parallel relation, wherein the polarizing means is a polarizing plate of a type in which a vibration direction of a transmission polarization component of the polarizing means rotates by 90xc2x0 when the passing direction of the transmitted light is changed, and each of the first and second phase plates has a wavelength dispersion of a retardation, such that a wavelength dispersion of a retardation composite value of the phase plates and the liquid crystal layer in a range from 400 (nm) to 700 (nm) of a wavelength of the transmitted light is set so as to be substantially approximated to a retardation curve expressed by the relational expression (0.5n)xcex, where n is an integer and xcex is a wavelength (nm) of the transmitted light.